Gas discharge lamps, such as conventional fluorescent lamps, offer substantial improvements over incandescent lamps, including higher energy efficiency and longer life. A drawback to fluorescent lamps, however, is that they can be difficult to control. This is due, in part, because they have "negative resistance." This means that the operating voltage decreases as current through the lamp increases. Therefore, circuits for supplying power to fluorescent lamps generally require a electronic ballast to maintain operating stability of the circuit and to provide an ability to dim the lamp.
During a typical manufacturing process for gas discharge lamps, the lamps are optimized to provide a maximum light output with a minimum amount of energy consumption. Different capacity gas discharge lamps having different lumen outputs are each designed for a different optimum voltage level. The benefits of high energy efficiency and long lamp life require that the ballast provide the gas discharge lamp with the optimum lamp voltage and which appropriately control the current for adjusting the light output of the lamp.
A conventional non-adjustable ballast provides a fixed lamp voltage and lamp current for a lamp with a specific lumen output. As a gas discharge lamp ages, however, the lamp deteriorates which causes the impedance of the lamp to increase. When such a lamp is operated with a non-adjustable ballast, this deterioration causes the lamp output to become increasingly dim over time. Accordingly, even though the non-adjustable ballast is initially optimized for the particular lamp, over time, the lamp output becomes increasingly dim and efficiency decreases.
A prior alternative to a conventional non-adjustable ballast is an adjustable fixed ballast. The adjustable fixed ballast allows the lamp current and lamp voltage to be adjusted by the user in an attempt to optimize a particular gas discharge lamp for a specific light output intensity. This allows gas discharge lamps of different capacities to be used in conjunction with identical ballasts. However, as stated above, the impedance of gas discharge lamps increases over time. Thus, over time, the gas discharge lamp will produce an increasingly dimmer light output and efficiency decreases. Therefore, optimization will be lost unless the user re-adjusts the ballast.
An approach to some of the problems associated with an adjustable fixed ballast is an electronic self-adjusting ballast. A common technique by which such a self-adjusting ballast regulates a gas discharge lamp is by sensing and controlling the current in the lamp. One problem with regulating only the lamp current is that the light output of the lamp is more closely related to the arc power of the lamp than to the lamp current. The arc power is equal to the product of lamp current and lamp voltage. Lamp voltage, however, is dependent on the temperature of the lamp. Therefore, if only current is regulated, the arc power and, hence, light output, will vary with the temperature of the lamp.
Another problem associated with gas discharge lamps is safety. When the gas discharge lamp is near the end of its useful life, the gas discharge lamp can continue to operate in a condition of partial rectification. When operating in partial rectification, there is a high cathode fall voltage in the region of a depleted cathode. Accordingly, operation in partial rectification causes excessively high power dissipation in the region of the depleted cathode. Further, when only lamp current is regulated, increases in the impedance of the lamp caused by aging results in increased power dissipation. As a result of these factors, portions of the gas discharge lamp can reach excessive temperatures. This can present a dangerous fire hazard and can cause the glass envelope of the lamp to shatter. This can pose an immediate safety hazard for persons in the vicinity of the lamp.
Although gas discharge lamps tend to be more efficient than their incandescent counterparts, it is advantageous for gas discharge lamps to operate in a dimmed mode. By operating in a dimmed mode, the light intensity from the gas discharge lamp can be adjusted according to the needs or tastes of the user. Unfortunately, prior control circuits for gas discharge lamps, especially small diameter lamps such as the T4, generally cannot operate in a dimmed mode below approximately 40% of the lamps' rated illumination output without the lamp extinguishing itself or flickering excessively.
A prior art electronic ballast and network for gas discharge lamps is described in U.S. Pat. No. 5,315,214 and shown in FIG. 1. FIG. 1 illustrates a prior art circuit which controls the illumination intensity of the lamp by controlling the current passing through the lamp. This prior art circuit also shuts off the lamp circuit when the lamp voltage exceeds a preselected threshold. Further, this prior art circuit utilizes a low pass filter at the output lamp network to allow the lamp to be dimmed. These features of operating of the circuit shown in FIG. 1 are disadvantages for the following reasons. Because the lamp current remains constant, the illumination intensity of the lamp will vary with impedance changes caused by aging of the lamp. Further, by sensing lamp voltage to determine when to shut down, in the case of a removed or unlit lamp, this prior art lamp circuit does not protect the lamp from circumstances when the lamp current remains constant and the lamp voltage rises thus causing excess power to dissipate into the lamp. Finally, this prior art lamp circuit does not allow the lamp, especially a small diameter lamp such as the T4, to be dimmed below approximately 40% without extinguishing itself or excessively flickering because of a high quality factor Q lamp network.
Therefore, what is needed is a control circuit for a gas discharge lamp that overcomes these disadvantages.